As a lithium ion secondary battery separator (sometimes abbreviated as “separator” hereafter) for a lithium ion secondary battery (sometimes abbreviated as “battery” hereafter), a porous film composed of a polyolefin resin such as polyethylene and polypropylene has been conventionally used. However, the resin-based porous film has had the problem that when a battery generates heat abnormally, the film melts and shrinks, and loses a function of separating positive and negative electrodes, which leads to a problem of causing a serious short circuit.
As a separator that is hard to melt and shrink even when a battery generates heat abnormally, there has been proposed a separator produced by coating various inorganic pigments onto a polyethylene terephthalate (PET) fiber-containing non-woven fabric base material for a lithium ion secondary battery separator (sometimes abbreviated as “non-woven fabric base material” hereafter) (for example, see Patent Documents 1-3).
Patent Document 1 describes a non-woven fabric base material comprising a fiber having a fiber diameter of 0.1 to 10 μm. Patent Document 2 describes a non-woven fabric base material comprising a crystallized PET fiber and a PET binder fiber, in which a short fiber having an average fiber diameter of 3 μm or less is contained as an essential component. Patent Document 3 describes a non-woven fabric base material comprising a crystallized PET fiber and a PET binder fiber, in which a fiber having a fiber length of 2 mm or less is contained as the crystallized PET fiber. However, each of non-woven fabric base materials disclosed in Patent Documents 1-3 has such problems that when it is intended to produce a separator whose thickness is small, the non-woven fabric base material is easy to be wrinkled during coating a coating liquid containing an inorganic pigment, which results in a decline in the productivity of the separator. For example, in Examples 2a to 2u in Patent Document 1, in order to produce a separator from a non-woven fabric base material having a thickness of 13 μm, there is used a complicatedly structured apparatus in which a non-woven fabric base material is conveyed while supporting said base material by a belt. Further, a coating liquid containing an inorganic pigment is coated on the base material at an extremely low line speed of 8 m/hr. In addition, there are cases where the coating liquid may penetrate though the base material, or there are cases where it may be difficult to achieve both low internal resistance and high tensile strength.
In addition, in each of the separators disclosed in Patent Documents 1-3, when the melting of a PET binder fiber is advanced to enhance the strength of the non-woven fabric base material, the PET binder fiber fills up pores inside the non-woven fabric base material, which lead to a problem that electrolyte retention worsens and a problem that the resistance of the separator becomes high.